Technical Field
The present invention relates to a method of compensating for frequency drift of a reference source in an interferometer employed to generate spectral data from unknown samples and to a Fourier Transform Interferometer based spectrometer instrument operating according to this method.
Related Art
Interferometric spectroscopy techniques are widely employed in the determination of chemical and compositional properties of unknown samples. Spectrometer instruments that operate according to these techniques do so generally by generating an interference pattern and detecting the effects on this pattern of an unknown sample of interest placed in the path of the interfering energy beams (or sometimes in the path of the energy beam before the interference pattern is generated). The so obtained interference data, so-called ‘interferogram’, observed in the time or position domain is then subjected to a numerical transformation in an arithmetic unit associated with the spectrometer instrument into information in the frequency or wavelength domain using Fourier Transformation. Such spectrometer instruments are therefore often referred to as Fourier Transform (FT) interferometers. Differences in the chemical and/or compositional properties of the unknown samples may then be correlated with the wavelength dependent intensity variations of the collected data. This is achieved typically from the application of a suitable calibration model to that data in the associated arithmetic unit.
One of the most common types of FT interferometer is the Michelson interferometer. This operates to generate the required interference pattern by separating incident energy from a probe energy source into two beams of more or less equal intensity using a beamsplitter; reflecting each of these beams from associated mirrors so as to have them recombine at the beamsplitter; moving one or both mirrors so as to create a variable path difference between the incident beams; and monitoring the interference pattern (interferogram) at a detector due to the path difference changes. By making measurements of the signal at many discrete and preferably equidistant positions of the movable mirror(s) the spectral information can then be reconstructed from the so collected interferogram by applying FT techniques in the arithmetic unit associated with the spectrometer.
As is well known for a Michelson type interferometer, the intensity of the interferogram at a particular path length difference between the two beams, so called retardation, may be expressed as a sum of cosine functions of the retardation. Each spectral element (or frequency) of the probe beam contributes to every point of the interferogram with the contribution (or weighting) of each element being unique for each point (assuming a single sided interferogram). The retardation is zero when the distance between the beamsplitter and each mirror is equal. This generates the so-called center-burst of the interferogram.
In order to reduce computational load the well known Fast Fourier Transform (FFT) technique is commonly employed in modern spectrometers employing Michelson type FT interferometers. Critical to the application of FFT techniques is the precise knowledge of the retardation at any time. In such Michelson type FT interferometers the movement of the mirror (or mirrors) is therefore precisely monitored. This is most usually done using a reference energy source which emits an essentially monochromatic radiation of known wavelength. This is typically a laser source which is configured to emit energy of known wavelength along a path through the interferometer which is substantially similar to that path traversed by the energy from the probe energy source. The resulting essentially single frequency oscillatory interference pattern that is detected by the detector is dependent on the relative position of the mirror (s) and the wavelength of the laser emission. Hence as the wavelength of the laser is, at least in theory, accurately known then the position of the moving mirror may be accurately determined or monitored. Thus, this oscillatory signal at the detector is employed to control or register the collection of the interference data at accurately known and equidistant retardation values. This may be achieved, for example, by triggering data collection at the zero amplitude crossing or with other periodically occurring features of the laser interferogram.
As can be appreciated, a variation in the emission wavelength of the laser emission between the collection of interference data at different instances for the same instrument or between different instruments will trigger data collection equidistantly but at slightly different distances. This will give rise to a phase shift in the interferograms collected at these different instances. This will manifest as a difference in the frequency or wavelength scale of the Fourier transformed collected data and ultimately to differences in the chemical and/or compositional properties of unknown samples which are to be correlated with the frequency or wavelength dependent intensity variations of the collected data in the associated arithmetic unit.
In order to mitigate this problem and correct for such phase differences caused by the reference source drift it is well known to standardize FT interferometer based spectrometers at intervals during their operational use. Typically in such a standardization event, such as is disclosed in U.S. Pat. No. 5,933,792, a measurement is made by the interferometer on a reference sample and the interferogram or Fourier transformed spectral data is compared in the arithmetic unit of the spectrometer instrument with a desired interferogram or transformed spectral data for that reference sample. The arithmetic unit then operates to generate parameters based on the comparison which describe the transition of the measurements for the reference sample to those of the desired measurements and which when applied to measurements on an unknown sample will transform those measurements to generate standardised measurements, corrected for frequency drift.
By this means information obtained for a sample using one instrument will be identical to that obtained for the same sample by any other instrument of the same type. Moreover, it is intended that such standardization will correct for the above described temporal variations in the same instrument.
Unfortunately the variations or drift in the wavelength of the reference energy, typically reference laser, source in the same instrument often occur much more frequently than the interval between standardization events for that instrument so that the above described standardization events only partially solve the problem.
This is particularly the case when solid state emission sources are used as the reference. These sources are often more susceptible to environmental variations than the helium/neon laser which has been traditionally employed as a reference. Frequent temperature drift is seen as a particular problem for these solid state sources and often relatively expensive temperature stabilization units are included in modern Michelson type FT interferometers in order to combat this.
It is an aim of the present invention to combat frequency drift In FT interferometers without the need for accurate temperature stabilization of the reference source.
According to the present invention there is provided a method of compensating for frequency drift in an FT interferometer based spectrometer instrument comprising the steps of:                (a) obtaining into an arithmetic unit of the spectroscopic instrument data representing a reference interferogram collected in response to a trigger signal having been generated in dependence on the emission frequency of a reference energy source to reflect a position of a moving optical element of the interferometer; and        (b) subsequently obtaining into the arithmetic unit data representing a target interferogram recorded by the FT interferometer; characterised in that the method further comprises the steps of: (c) comparing in the arithmetic unit the data representing the reference interferogram and the data representing the target interferogram to determining a phase shift between the interferograms in at least one region away from center-burst; (d) generating in the arithmetic unit a mathematical transform dependent on the determined shift or shifts; and (e) applying the mathematical transform to control the operation of the spectrometer to generate data representing a frequency stabilized interferogram of an unknown sample recorded by the FT interferometer.        
The transform may be generated to maximize a phase correlation between the reference and the target interferogram data over an interferogram region of interest.
The transform may be applied in the arithmetic unit to mathematically correct the interferogram recorded for the unknown sample
Conveniently, each interferogram may be initially phase corrected to ensure that there will be zero phase shift of every contributing frequency component for each of the two interferograms at center-burst. This may, for example, be performed in the arithmetic unit by, for each of the two Fourier transformed interferograms firstly determining their associated power spectrum (being the length of the complex spectrum after the application of FFT to the interferogram), then performing a reverse Fourier transformation to generate a phase compensated interferogram for each of the two originally measured interferograms. In this manner the newly generated interferograms are such that all contributing frequencies will have a zero shift at center-burst. This advantages that any phase difference away from the center-burst is will be maximised for a given drift in reference laser frequency and that the two interferograms may be reliably phase aligned at center-burst.
Since the frequency drift will actually cause a ‘stretching’ of the interferogram preferably the transform Is also made dependent on a distance in the interferogram from center-burst, such as a fraction or percentage of the position of the moving optical element, where the size of the fraction or percentage is calculated from the determined shift or shifts, for example calculated as a relative shift or shifts.